Light olefins, defined herein as ethylene, propylene, butylene and mixtures thereof, serve as feeds for the production of numerous important chemicals and polymers. Typically, light olefins are produced by cracking petroleum feeds. Because of the limited supply of competitive petroleum feeds, the opportunities to produce low cost light olefins from petroleum feeds are limited. Efforts to develop light olefin production technologies based on alternative feeds have increased.
An important type of alternate feed for the production of light olefins is oxygenate, such as, for example, alcohols, particularly methanol and ethanol, dimethyl ether, methyl ethyl ether, diethyl ether, dimethyl carbonate, and methyl formate. Many of these oxygenates may be produced by fermentation, or from synthesis gas derived from natural gas, petroleum liquids, carbonaceous materials, including coal, recycled plastics, municipal wastes, or any organic material. Because of the wide variety of sources, alcohol, alcohol derivatives, and other oxygenates have promise as an economical, non-petroleum source for light olefin production.
The catalysts used to promote the conversion of oxygenates to olefins are molecular sieve catalysts. Because ethylene and propylene are the most sought after products of such a reaction, research has focused on what catalysts are most selective to ethylene and/or propylene, and on methods for increasing the life and selectivity of the catalysts to ethylene and/or propylene.
The conversion of oxygenates to olefins generates by-products whose presence is undesirable for subsequent applications of the collected olefins. Although the separation of many oxygenates, e.g., ketones and aldehydes, from hydrocarbons such as olefins can be capably handled by existing commercial processes, the separation of other oxygenates, e.g., dimethyl ether (DME) can be problematic.
DME is an oxygenate impurity formed during the conversion of methanol into light olefins which can act as a poison to downstream olefin polymerization catalysts, especially metallocene catalysts. Removal of DME from oxygenates to olefins product streams is thus highly desirable. Unfortunately, such removal can be difficult given, inter alia, DME's physical characteristics similar to certain lower olefins, e.g., its similar volatility to propylene. Separation of DME from propylene by distillation, e.g., using a C3 splitter, requires a super fractionation column requiring significant capital investment. Alternatively, DME's difference in solubility from lower olefins can be exploited by using a water wash to remove DME from an olefinic product stream. Unfortunately, given DME's non-polar characteristics, an extensive volume of water would be required in a water wash tower so employed. Given these difficulties it would be desirable to provide a process for removing DME from olefin-containing streams such as those obtained by conversion of oxygenates to olefins, which does not require superfractionation or water washing.
Methods for recovering and recycling dimethylether (DME) from a methanol-to-chemical conversion reaction using a DME absorber tower are disclosed in U.S. Pat. No. 4,587,373 to Hsia. Stud. Surf. Sci. Catal. (1985), 20 (Catl. Acids Bases), 391–8, discusses low temperature conversion of dimethyl ether over Pt/H-ZSM-5 in the presence of hydrogen by a bifunctionally catalyzed reaction. Stud. Surf. Sci. Catal. (1993), 77 discusses hydrogenation of oxygenates such as dimethyl ether over a Ni/Al2O3 catalyst to form methane. U.S. Pat. No. 5,491,273 to Chang et al. discloses conversion of lower aliphatic alcohols and corresponding ethers to linear olefins over large crystal zeolites, e.g., ZSM-35 containing a hydrogenation component of Group VIA and Group VIIIA metals. DE3210756 discloses a process for converting methanol and/or dimethyl ether feed to olefins by reacting the feed over a pentasil type zeolite catalyst, separating C2–C4 olefins, methane and water from the reaction product and catalytically hydrogenating the remaining components over Co—Mo supported on alumina, optionally preceded by hydrogenation over a Group 8 noble metal for polyunsaturated, non-aromatic compounds. U.S. Pat. No. 4,912,281 to Wu discloses converting methanol or methyl ether to light olefins in the presence of hydrogen and ZSM-45 which is highly selective to C2–C4 olefins, especially ethylene. DE2720749 discloses converting lower aliphatic ethers to hydrocarbons in the presence of amorphous, non-acid-activated Al silicate. U.S. Pat. No. 4,625,050 to Current discloses the use of carbonylation to convert dimethyl ether to methyl acetate and ethanol (as well as minor amounts of methyl formate and propanol) over hydrogen and CO in the presence of heterogeneous NiMo catalyst on an alumina support. EP-229994 discloses the removal of DME as an impurity (1–500 wppm) of olefinic hydrocarbon feedstock by passing the feedstock through an adsorbent mass of crystalline zeolite molecular sieve having the crystal structure of faujasite at 0°–60° C. and 0.15–500 psia to selectively absorb DME. All of the above references are incorporated herein by reference in their entirety.
In addition to DME, light olefin products, especially those generated by steam cracking or derived from oxygenated feedstocks, can contain unsaturated by-products such as acetylene, methyl acetylene (MA) and propadiene (PD). Making olefins from oxygenated feedstocks produces a unique effluent stream that must ultimately be separated and purified to produce the high purity olefin products currently desired. These unsaturated by-products poison polyolefin catalysts, and therefore must be almost completely removed from olefin product streams. For ethylene, current manufacturing specifications can require acetylene levels to be under 0.5 mole ppm. For propylene, current manufacturing specifications can require methyl acetylene and propadiene levels to be under 2.9 mole ppm.
Catalysts for selectively hydrogenating highly unsaturated compounds are known in the art. For example, U.S. Pat. No. 6,084,140 to Kitamura et al. discloses a palladium and alumina catalyst for hydrogenating highly unsaturated hydrocarbons in olefin streams from steam cracking processes. The catalyst can hydrogenate acetylene, methyl acetylene, and propadiene, with only limited hydrogenation of the olefin products. U.S. Pat. No. 4,367,353 to Inglis discusses a hydrogenation process using a supported palladium catalyst. The process involves first fractionating the hydrocarbon streams before hydrogenating whereby hydrogen is removed. Hydrogen is added during a subsequent hydrogenation step, allowing for greater control of the extent of hydrogenation. Because the concentration of unsaturated by-products acetylene, methyl acetylene, and propadiene can increase to three times their initial amounts during the purification of the hydrocarbons by fractionation, the concentration of acetylene, methyl acetylene and propadiene must be three times lower in front-end hydrogenation than in tail end hydrogenation. Achieving this greater purity results in greater loss of olefin products during the hydrogenation process. U.S. Pat. No. 5,837,217 to Nielsen et al. discloses preparation of hydrogen rich gas from a feed stock of dimethyl ether and steam, wherein the dimethyl ether is reacted with steam in the presence of i) an ether hydration catalyst such as acidic zeolites, e.g. HZSM-5, and ii) a methanol decomposition catalyst, e.g., Cu—Zn-alumina. U.S. Pat. No. 6,413,449 to Wieland et al. discloses a catalyst comprising palladium/zinc alloy and zinc oxide as catalytically active components useful for the steam reforming of alcohols, e.g., methanol to produce hydrogen-rich gas. All of the above references are incorporated herein by reference in their entirety.
Given the difficulties presented in separately removing by-products DME and the unsaturated compounds methyl acetylene, propadiene and acetylene from olefinic product streams, particularly those product streams from steam cracking and oxygenate to olefins processes, it would be advantageous to remove at least one or more of these by-products with techniques that do not require dedicated equipment for superfractionation, water washing, etc. Moreover, it would be advantageous to at least partially remove these by-products using equipment commonly found in existing olefin plant recovery trains, e.g., hydrogenation reactors. Accordingly, it would be particularly advantageous to remove DME along with the hydrocarbon impurities acetylene, methyl acetylene, and propadiene from the product stream using the same equipment.